1. Field of the Invention
The present invention relates to a process of the preparation of methylene bridged polyphenylene polyisocyante, and particularly relates to a fractionation process of a reaction mixture containing the polyisocyanate.
Methylene bridged polyphenylene polyisocyanate is a very reactive compound and widely utilized for polyurethane products such as foams, elastomers, adhesives and paints.
The present invention can conduct various modes of fractionation on methylene bridged polyphenylene polyisocyanate having the above many uses depending upon the kind and purity of the polyisocyanate and the variety and amount of impurities The above reaction product can hence be fractionated so as to obtain reaction materials which satisfy properties required for specific uses.
2. Disclosure of the Related Art
Methylene bridge polyphenylene polyisocyanate is prepared by phosgenating a polyamine mixture formed by condensation of aniline and formaldehyde. The product contains various kinds of diphenylmethane diisocyanate (hereinafter referred to as MDI) and can provide by distillation an isomer mixture of 2,4'-diphenylmethane diisocyanate (hereinafter referred to as 2,4'-MDI) and 4,4'-diphenylmethane diisocyanate (hereinafter referred to as 4,4'-MDI).
The residue obtained by the distillation also has various uses and thus preparations are generally made so as to suitable for the uses by mixing these fractionated products in an adequate proportion. However, phosgenation in preparing the MDI mixture inevitably generates impurities having acidity and hydrolyzable chlorine. These impurities have been known to lower the reactivity of the isocyanate group in the preparation of polyurethane products.
Conventionally known processes for reducing the acidity and hydrolyzable chlorine (hereinafter referred to as HC) contents are based on the treatment of the MDI mixture with an organometallic compound at high temperature. These processes, however, are not satisfactory because the organometallic compound is required in a large amount, remains in the product and leads to formation of tarry materials in the high temperature treatment.
On the other hand, separation processes by distillation, extraction and crystallization, respectively, have been conventionally known for the separation of the MDI isomer mixture. The separation process by distillation causes polymerization of isocyanate due to heating and decreases the yield of 4,4'-MDI. The separation process by extraction has a disadvantage of requiring solvent removal. The separation process by crystallization divides the isomer mixture into a mother liquor and a crystalline product and affords 4,4'-MDI as a high purity crystal. The process, however, cannot sufficiently reduce the acidity and HC content of the resulting 4,4'-MDI.